This is basically just adapting old code to the new interface. A lot
of the functions are still stubs, but the basic stuffs is working.
What remains to be done:
* Implement gl_image_op. (It should do the operation lazily, only update
the flags on the texture. Actual processing will be delayed until composition.)
* Implement gl_copy. Now it just return the same image after incrementing the
refcount. It should actually copy the image data structure so it can have a
separate set of flags.
Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>
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